This series started out as 2 mix CDs sets of DJs in different clubs around the world remixing songs of artists local to the area. It's been fairly popular since the artists are pretty damn good at spinning that trance, and it's a good look at music around the world. Recently, they've been growing beyond the Artist - Location series and showcasing less-known artists. As proj2501 mentions, calling them Underground is a cheap attempt to seem like they're not mainstream, but it's still good music. The various albums, artists and locations of the Global Underground (plus comments on the ones I've heard):

Classic GU:

GU001 - Tony De Vit - Tel Aviv
GU003 - Nick Warren - Prague
GU004 - Paul Oakenfold - Oslo
GU005 - Tony De Vit - Tokyo
GU006 - John Digweed - Sydney (Wow. Amazingly good.)
GU007 - Paul Oakenfold - New York
GU008 - Nick Warren - Brazil
GU009 - Sasha - San Francisco
GU010 - Danny Tenaglia - Athens
GU011 - Nick Warren - Budapest
GU012 - Dave Seaman - Buenos Aires
GU013 - Sasha - Ibiza
GU014 - John Digweed - Hong Kong (Good subtle trance that grows on you.)
GU015 - Darren Emerson - Uruguay

New GU:
Arrivals - Various Artists - GU Sampler
Nu Breed - Anthony Pappa

Note: GU002 was never a record, but instead a limited edition T-shirt.

Global Underground is a series of techno/house/trance compilations masquerading as DJ sets. The Global Underground folks digest the setlists of famous DJ's all over the world, then stuff as many as they can onto two discs, mix them together with Pro Tools, and put together some flashy graphics and annoying liner notes.

Then they sell this as "underground" music on sale at Wal-Mart. The DJ is the new rock star. *le sigh*